The present invention relates to a method for manufacturing phase gratings which diffract light beams according to phase control, and more particularly, phase gratings having combined characteristics of a pattern modification type and a refraction modification type phase grating.
In general, a phase change P which is caused when light beams of wavelength .lambda. travel a distance D within a light medium of a refractive index n is shown by the equation ##EQU1## When a periodic distribution of the phase variation P is caused on an exit plane of the light medium due to variations in the distance D and the refractive index n, the light beam are diffracted in such a direction that light beam components with different phase changes P amplify each other by phase matching.
Conventional phase gratings are broadly divided into the following two categories. One, as shown in FIG. 3, is a pattern modification type phase grating 1A which comprises a transparent plate 2A including regularly arranged convex-concave portions a, b for controlling phases of light beams and the other, as shown in FIG. 4, is a refraction modification type phase grating 1B which comprises a transparent plate 2B including regularly arranged layers a', b' of different refractive indexes, respectively, for controlling phases of light beams which pass therethrough.
The phase grating 1A shown in FIG. 3 is in common use for light branching elements, diffraction gratings for dividing light beams, Fresnel lenses and the like. With the phase grating 1A, however, in order to amplify diffracted light beams other than zero-order diffracted light beams by suppressing the latter, it is necessary to increase the difference in level of regularly arranged convex-concave portions, so that a high degree precision working is required when a diffraction angle is increased by minimizing the pitch of the convex-concave portions, resulting in difficulty in manufacturing phase gratings 1A.
In the phase grating 1B shown in FIG. 4, a method for amplifying diffracted light beams other than zero-order diffracted light beams by suppressing the latter is to increase the difference in refractive index between regularly arranged layers or thickness of the grating 1B. However, the increasable range of difference in refractive index is comparatively limited in practice, so that the phase grating 1B is disadvantageous in that its thickness is extremely increased as compared with the pattern modification type phase grating 1A.
We have already proposed a phase grating 1, as shown in FIG. 1, of a combination pattern-refraction modification type which positively utilizes an optical multiplication effect on the basis of the combined modifications of a pattern modification type phase grating 1A and a refraction modification type phase grating 1B.
Prior art in connection with formation of a difference in level and a difference in refractive index according to photoreaction are summarized only in the following two cases. The first case is shown in E. A. Chandross, C. A. Pryde, W. J. Tomlinson, and H. P. Weber; Appl. Phys. Lett., Vol. 24, No. 2 (1974), pp. 72 to 74. According to this, it is reported that after a film is formed on a pyrex substrate by a solution pile method, by adding a 16 weight percent of acrylic acid ethyl 2-(1-naphthyl) as a photoreactive dopant to a copolymer including a derivative of glycidyl methacrylate and methyl methacrylate, the dopant is dimerized by irradiating ultraviolet rays of wavelength 300 to 380 nm through a photomask and then non-reacted dopant is volatilized by heating for about one hour at temperatures of 100.degree. to 105.degree. C. in a nitrogen atmosphere, whereby the exposed portion increases in film thickness by about 15% and in refractive index by 0.8 to 1.0% as compared with a shaded portion.
The second case is shown in W. J. Tomlinson, H. P. Weber, C. A. Pryde, and E. A. Chandross, Appl. Phys. Lett., Vol. 26, No. 6 (1975), pp. 303 to 306. According to this, it is reported that it in the first case when a 13 weight percent of 2-naphthalenethiol is added as a photoresistant dopant, the dopant is bonded to a copolymer by irradiating ultraviolet beams, resulting in that an exposed portion increase in film thickness by about 10% and in refractive index by about 1.3% as compared with a shaded portion, whereby a ridge waveguide having a light propagation loss 0.3 dB/cm and a light coupler having a coupling intensity 15 dB/cm are manufactured.
These reports, however, are totally different from the present invention with respect to the subject matter, polymer material, photoreactive additive, film forming process, type of photoreaction, the upper limit of variations in film thickness and refractive index, and application. In addition, there is another report in which when a film of poly(methyl methacrylate) is irradiated by electron beams, film thickness increases and refractive index increases, whereby a diffraction grating is manufactured (H. Kotani, M. Kawabe and S. Namba, Japanese J. Appl. Phys., Vol. 18, No. 2 (1979), pp. 279 to 283). However, the report is independent of the present invention.